Method of making hardened gelatin films and resulting product



July 7, 1936- B. B. BURBANK 2,046,320

METHOD OF MAKING HARDENED GELATIN FILMS AND RESULTING' PRODUCT Filed Oct. 25, 1934 Atys.;

Patented July 1, 1.936

METHOD F MAKING FILMS HARDENED GELATIN AND RESULTING PRODUCT Benjamin n. Burbank, Brunswick, Mame, as! signor to'Teclmicolor Motion Picture Corporation, Hollywood, Calif., a corporation of Maine Application October 25, 193g, SeriaiNo. 749,961l

20 Claims. (Cl. 18-57) This invention relates to a method of preparing hardened gelatin lms, suitable for imbibition printing and the like, and to the resulting product.

In the art of hardening gelatine films or sury faces itvhas heretofore been found that the hardening liquor employed may advantageously be neturalized in order to provide an improved hardening action. The reason for neutralizing the hardening liquor is probably attributable to the fact'that the Aswellingiof pmteinsv depends u upon the chemical concentration of water molecules about the primary amino groups of. the protein molecule,iwhereas the same amino groups u participate materially in the hardening reaction with tanning agents. Iherefore, the water molecules and the hardening agent may be said.

K to compete in their'reactionwith theprotein molecule, and, since the amount of hydrate water 20 increases with the acidity, it follows that the hardening action is reduced by highacidity and the resulting abundancy ofmhydrate water; .con-

versely neutralization, and hence diminution of such tendency to hydration, is favorable to the g5 hardening reaction. It has also been found that the most favorable tanning action takes place if the hardening solution has a pH or hydrogen ion concentration near the isoelectric pointof the material to be hardened. However, the iso- 30 electric points of gelatin, and other proteins have values which could heretofore not be obtained in hardening solution such as chrome or alum solutionsof workable concentration, (and more particularly chrome alum solutions) since 35 the normal chrome salt or alum solutions precipitate before the desired neutralization is reached, the pH value at the point ofprecipitation depending uponlthe' concentration of the solution. The lower the concentration, the more 40 alkaline thesolution may be made without precipitation. For example, with chrome concentrationsof 1% and even less, precipitation may be practically complete at pH 5.3, whereas with higher concentrations, as for example l5 to 45' 20%, precipitation may begin at pH 3.5. The

desirable pH corresponding to the isoelectric point of protein could, heretofore, not be reached with concentration higher than 3 to 4%.

It is therefore the principal object of the 50 present invention to provide a method for neutralizing hardening liquors to anydesired degree, without incurring precipitation. As 'ap- .plied especially to the hardening of gelatin nlm, such as that used as blank lnlm for the printing 56 thereon of dye pictures from matrices (the soing with chrome hardeners, but it is expressly called imbibition process) some of the objects of the invention are to provide a method for obtaining a degree of gelatin hardness, which is an optimum for the purpose of such blanks, to pro' y vide such a method which permits hardening at 5 the isoelectric point of lthe gelatin, with an increased range of concentrations of the hardening reagent and to accomplish this hardening action with a minimum amount of the hardening agent.

In another aspect, it has been found by experimentation that for a given concentration of hardening liquor the hardened material requires and retains more of the hardening agent, the more acidic the hardening solution. Accordingly, the material requires a considerable time vto reach a stable condition, that is, constant hardness after manufacture, when it has been treated in .a relatively acidic hardening u bath. It is, therefore, another object of the presentinven- 26 tion to provide a method for hardening proteins, as for example gelatin, to adesired degree-of hardness without incurring serious changes of its qualities subsequent to .manufactura In still another aspect, it is sometimes desirable, especially in the production of the abovementioned blank lm for imbibition printing, to add to the hardening bath suchordinarily violent precipitants as suldes, sulfites and thiosulfates, and'it is a further object of thepresent invention tov provide a method which permits the adding to hardening solutions of such sulfur salts without danger of precipitation.

Other objects and'advantages of the invention will be apparent from the following detailed description which refers to the accompanying. drawing in which Fig.' 1 represents curves showing'the hardness and residual chromium contents of a gelatin nlm treated according to the invention with reference to the hydrogen-ion concentration of the hardening bath in which it hasbeen treated; and

Fig. 2 presents curves showing the variations in hardness obtainable with different concentrations of chrome alum solutions, prepared in accordance with this invention. I

The following description refers especially to the treatment of blank nlm for imbibition printunderstood that the new method is also applicable to the analogous treatment of other proteins.

As mentioned before, chrome solutions precipitate uponreaching a certain neutralization. For

example, the isoelectric point of gelatin as used in photographic films is approximately 4.7. But a sufllciently reactive concentration of the chrome alum bath is about 3 to 4% which has a pH value of about 3.6. If a neutralizing agent is added to a chrome alum-hardening bath of that concentration until a pH of 4.7 is reached the chromium 'component is precipitated until there remains only 1% or less chrome alum actually in solution, which concentration has been found unsuitable for hardening. If a ,chrome alum bath of 3 to 4% is first neutralized to a point just s ort of precipitation, a pH of about 3.4 to 3.9 can be'reached. But this is too low for rapid hardening. f

According to the present invention, tartaric acid, or a salt thereof, or glycerine, is added to the chrome hardening bath (consisting for example of a solution of chrome alum or chromic sulfate, preferably, but not necessarily in proportions governedby the chromium percentage, namely roughly one mol. tartaric acid to one atom chromium), and this mixture is subsequently brought to just the boiling point. Subsequent to this procedure, any amount of neutral-1 izer, as forA example ammoniumhydroxide or such alkaline substances as sodiur'n acetate, or even of vigorous precipitants as sodium sulfide, or sulfites and thiosulphates, can be added without causing precipitation. It is quite feasible in this manner to obtain 'a pH value of 9.0 with substantially any chromium-concentration.

Referring now to the figure, curve A shows the degrees of hardness which can be obtained by treating photographic gelatin with a 10% chrome alum solution of varying acidity, arbitrary units of hardness being plotted over pH values, whereby the hardness is expressed by the rate of dye penetration, as established by bathing treated gelatin films in a' standard dye solution for a given time, and by determining the amount of dye absorbed by means of optical measurements. The line B indicates the pH value of the chrome alum bath which can be obtained without the use of tartaric acid, and it indicates that the neutralization of the 10% chrome alum bath could heretofore not be conbeginning to precipitate at that point. 'Ihis is avoided by using the tartaric acid treatment according to the invention, and the curve A continues as indicated, a maximum being reached with hardening solutions having a pH of approximately 4.9, from which point the hardening drops again with increasing pH.

For different chromium concentrations, the hardness-pH-concentration curves have similar shapes, the limit line B moving towards the right with decreasing concentrations and the positions and values of the peaks of obtainable hardness also varying corresponding lto .varying values of pH of the liquor. Therefore, since the absolute degree of hardness depends upon' the chromium concentration, and since with the new method a pH close to that of the isoelectric point for gelatin may be had at any chromium concentration, it will now be apparent that the most favorable hardness can be attained with the mininum amount of chrome. I

' Curve C indicates the amount of residual chromium found in a gelatin film after treatment with tartaric acid treated chrome alum solution of a certain concentration, chromium units being plotted -overpH values. The curve shows that in a general way the residual chromium content decreases rapid1y, with rise .in pH values.' inthe pH rangefin which the so that it is possible to harden to a certain predetermined extent and to preserve the film at that hardness after the hardening treatment.

Further, as mentioned hereinbefore, the tartaric acid may be used to prevent precipitation of chromium when adding to the hardening bath suchl substances asvsuliides, sultes, or thiosulfates, whose use may be advantageous in the production of blank iilm for imbibition processes.

It has been found that the tartaric acid treatment according to this invention is applicable not only to chrome hardeners as chrome alum, chromic sulfate and other more or less complex chromium salts, but also to hardening solutions prepared with other chromic salts, such as sodium or ammonium alums, as well as potassium alum.

As an example for the commercial hardening of a blank film for imbibition printing, that is, a

tol

Celluloid lm covered with alayer of plain gelatin, the following embodiment of the invention is described. The hardening solution is prepared by 35 dissolving 525 grams of chrome alum Crz (S04) K2SO424H2O in 10 liters of water and by adding 150-200 grams of tartaric acid [2 (CH.OH.CO2H) l. 'Ihe vsolutionmay then be brought to a boil or .approximately to boiling temperature. Afterv amount of ammonium hydroxide necessary for tinued beyond a pH of about 3.6, the chrome this purpose is about -200 cubic centimeters.

If reducing compounds, such as the compounds of sulphur above mentioned, are to be employed, they may be added at this stage or lmay-be used in whole or in part as the alkaline reagents with which to adjust the `pH value of the treating liquor, in place of the ammonium hydroxide, for example. But such additions are made slowly and quickly dispersed inthe reagent solution so as to avoid the formation of local concentrations which tend to'precipitate the metal component of the reagent and the redissolving of such precipitates is dilcult and ,impractical if not impossible. The same prethe hardening of gelatin and the like, which the scope 70 comprises adding tartaric acid thereto and subsequently neutralizing the same, thereby permitting the neutralizing step to be effected over a wide range of pH values without precipitation of the hardening agent.

2. Method of treating reagent'solutions fo the hardening of' gelatin and the hke,` which comprises adding tartaric acid thereto, bringing the solution to a boil, and neutralizing said liquor, thereby permitting the neutralizing steps to be effected over a wide range of pH values `without precipitation of the hardening agent.

3. Method of treating alum solutions for hardening gelatin and the like, which comprises addtration of the solution.

4. Methodl of hardening gelatin iilms or the like with'basic alum solutions, which comprises adding tartaric acid to the hardening solution, bringing the solution to a boil, neutralizing the solution approximately to theisoelectric point of' the gelatin, and subsequently bathing the gelatin iilm in the solution.

5. Method of hardening gelatin iilms with a chrome alum hardening solution which comprises adding tartaric acid thereto, in the prof portions of approximately `oie` m'ol. tartaric acid to one mol. oi' chrome alum, bringing the solution approximately to a boil, neutralizing with agent.

'1. Method'of treating alum solutions-.for the hardening of gelatin and the like which com-l prises adding an organic acid to the solution,I

bringing approximately to a boil, and subsequently neutralizing with a soluble sulfide without precipitation' of the hardening agent.

8. Method of treating alum solutions for the hardening oi' gelatin and the like which com.

prises adding an organic acid to the solution, bringingv approximately to a boil, and subsequently neutralizing' with a soluble sulilte without precipitation of the 'hardening agent.

9. Methodof treating alum solutions for the hardening of gelatin and the like which 'comprises adding an organic acid-to the solution, bringing approximately to a boil, and subsequently neutralizing with a soluble thiosuliate without precipitation of the hardening agent.

10. Method of obtaining an optimum hardness oi gelatin nlm with a hardening solution containing a minimum, amount o! chromium, which comprises dissolving the chrome hardening agent in water, adding tartaric acid in approximately equimolecular proportions. neutralizing to approximately the isoelectric point of the gelatin of saidv iilm, and bathing said nlm in said solution, the amount of hardening agent retained being restricted to a degree whichproduces the .maximum hardness at the isoelectric point of said gelatin. f

11. Method of obtaining an optimum hardness of gelatin film with a hardening solution conl taining a minimum amount of chromium, which comprises dissolving the chrome hardening agent in water, adding tartaric acid in approximately equimolecular proportions, neutralizing to a pH slightly above th isoelectric point of the.gela tin of said liilm, and bathing said ilm in said solution, the amount of hardening agent re tained being restricted to a degree ,which produces the maximum hardness at the isoelectric point of said gelatin.

12. Method of making illm blanks for imbibition printing comprising as a step subjecting the lm `to a hardening reagent while controlling the vpl-I to approximate the isoelectric point of 20.`

kgelatin in the presence of an organic acid radical. i

13. Method of making lm blanks for imbibition printing comprising as a step subjecting the iilm\to a hardening reagent while con. 25 trolling the pH to approximate the isoelectric point of gelatin in the presence of tartaric acid.

14. Method of making iilm blanks for .imbibi- .tion printing comprising as a step subjectingthe film to a hardening reagent while controlling 30 the pH to approximate the isoelectric point of gelatin in the presence of acetic acid.

15. Method oi making film blanks for imbibition printing comprising as a step subjecting the iilm` to a hardening solution of alum while' controlling the pH to approximate the isoelectric point oi.r gelatin in the presence of an organic acid radical.

16. Method oi.' making nlm blanks for imbibiti'on printing comprising as a step subjecting the lm to a'hardening solution oi' chrome alum while controlling the p H to approximate the isoelectric point of gelatin in the presence' of tartaric acid.

`1'7. Methodof making nlm blanks for imbibi-` tion printing comprising as a step subjecting the i'ilm to a hardening reagent lwhile-controlling the pH to approximate the maximum hardening point for gelatin, in the presence'of an organic acid radical.4 j

18. A hardening solution, iorvhardening gelatin and the like, comprising an alum hardeningv i reagent andan organic acid radical and characterized by having a pH value approximating the isoelectric point of gelatin. y

19. A hardening solution.,. for hardening lgelatin and the like. comprising a chrome alum hardening reagent and an organic acidradical and. characterized by having a pH value.ap proximating the isoelectric point of gelatin. 60 20. A hardening solution, for hardening gelatin` and the like, comprising a chrome alum hardening reagent and tartaric acid radical and. characterized by having a pH -value approxi mating the isoelectric'point of gelatin.

B'. BURBAN. 

